Abstract. Remote sensing of cloud condensation nuclei (CCN) would help evaluate the
indirect effects of tropospheric aerosols on clouds and climate. To assess
its feasibility, we examined relationships of submicron aerosol composition
to CCN activity and optical properties observed during the MILAGRO/INTEX-B
aircraft campaigns. An indicator of CCN activity, κ, was calculated from
hygroscopicity measured under saturation. κ for dry 100 nm particles
decreased with increasing organic fraction of non-refractory mass of
submicron particles (OMF) as 0.34–0.20×OMF over Central Mexico and
0.47–0.43×OMF over the US West Coast. These fits represent the critical dry
diameter, centered near 100 nm for 0.2% supersaturation but varied as
κ(−1/3), within measurement uncertainty (~20%). The decreasing
trends of CCN activity with the organic content, evident also in our direct
CCN counts, were consistent with previous ground and laboratory observations
of highly organic particles. The wider range of OMF, 0–0.8, for our
research areas means that aerosol composition will be more critical for
estimation of CCN concentration than at the fixed sites previously studied.
Furthermore, the wavelength dependence of extinction was anti-correlated
with OMF as −0.70×OMF+2.0 for Central Mexico's urban and industrial
pollution air masses, for unclear reasons. The Angstrom exponent of
absorption increased with OMF, more rapidly under higher single scattering
albedo, as expected for the interplay between soot and colored weak
absorbers (some organic species and dust). Because remote sensing products
currently use the wavelength dependence of extinction albeit in the column
integral form and may potentially include that of absorption, these regional
spectral dependencies are expected to facilitate retrievals of aerosol bulk
chemical composition and CCN activity over Central Mexico.